1. Field of the Invention
The present invention relates to voltage output devices with operational amplifiers, and more particularly, to a technique of compensating for offset in output from an operational amplifier.
2. Description of the Related Art
Operational amplifiers (differential amplifiers) have offset caused by differences in characteristics between input transistors. That is, although the operational amplifier is a circuit component for amplifying the potential difference between the input terminals, the operational amplifier provides non-zero output voltages even when the same voltage is applied to both the input terminals. Such offset in the operational amplifier has been eliminated, for example, using conventional techniques as follows. Disclosed in Japanese Patent Kokai No. 2005-33541 (Patent Literatures 1) is an offset voltage compensation circuit, which includes a comparator for comparing the output voltage from the operational amplifier with a reference voltage to detect an offset voltage of the operational amplifier. The circuit also includes a counter for incrementing or decrementing count values by a value corresponding to the magnitude of the offset voltage, and a register for varying the resistance value of a variable resistor, which constitutes the operational amplifier, on the basis of the count value of the counter.
Disclosed in Japanese Patent Kokai No. H11-305735 (Patent Literature 2) is a differential amplifier circuit that constitutes a driver for driving a liquid crystal display panel. This differential amplifier circuit is configured to operate while alternately switching between a pair of input transistors, thereby eliminating offset components when averaged over time.
Japanese Patent Kokai No. H11-305735 (Patent Literature 3) and Japanese Patent Kokai No. 2008-258725 (Patent Literature 4) relate to a method to be employed for a driver circuit for driving liquid crystal display panels. The method includes the steps of detecting the offset of an operational amplifier by comparing the output voltage of the operational amplifier with a reference voltage, and sequentially connecting gain adjustment capacitors to the operational amplifier until the offset disappears.